The present invention relates generally to four color, single pass color printing systems and, more particularly, to a color printing system consisting generally of a raster output scanner (ROS) optical system and a plurality of tri-level engines arranged in tandem for producing full process color images as well as highlight color images.
In the practice of conventional bi-level xerography, it is the general procedure to form electrostatic latent images on a charge retentive surface such as a photoconductive member by first uniformly charging the charge retentive surface. The electrostatic charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a bi-level latent charge pattern on the imaging surface where the high charge regions correspond to the areas not exposed by radiation. One level, usually the higher of the two levels of the charge pattern, is made visible by developing it with toner. Development of the lower level charge is commonly referred to as reversal development. The toner is generally a colored powder that adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface, or is transferred to a receiving substrate such as plain paper, to which it is fixed by suitable fusing techniques.
In tri-level imaging, unlike conventional xerography, the image area contains three voltage levels which correspond to two image areas and to a background voltage area intermediate the two image areas. One of the image areas corresponds to non-discharged (i.e. charged) areas of the photorecptor while the other image areas correspond to discharged areas of the photorecptor.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
U.S. patent application Ser. No. 07/632,298 filed in the name of George J. Roller on Dec. 21, 1990, now U.S. Pat. No. 5,194,351, discloses a xerographic method and apparatus capable of achieving a large gamut of colors using the tri-level, highlight color process. Tri-level images are formed within pixel distance of a prior developed image. These images are developed with one of two different color toners followed by recharging of the charge retentive surface and a second exposure to form more tri-level images which are selectively developed using two different color toners which are also different in color from the other toners.
U.S. patent application Ser. No. 07/923,648 file on Aug. 3, 1992, now U.S. Pat. No. 5,223,906, in the name of Ellis D. Harris relates to a four color toner, single pass color printing system consisting generally of a raster output scanner (ROS) optical system and two tri-level xerographic units in tandem. Only two of the three subtractive primary colors of cyan, magenta and yellow are available for toner dot upon toner dot to combine to produce the additive primary colors. The resulting color printing system is able to produce pixels of black and white and five of the six primary colors, with pixel next to pixel printing producing all but the strongest saturation of the sixth primary color, an additive primary color. The color printing system uses either four color toners or a black toner and three color toners.
U.S. Pat. No. 4,903,048 granted to Steven J. Harrington on Feb. 20, 1990 relates to simulated color imaging using gray level patterns produced from two differently colored materials by employing fine patterns of dots positioned next to each other. The dots blend with the background and yield a gray or colored appearance when seen from a distance. The imaging process utilizes ink pattern designs in conjunction with registered two-color imaging to thereby form simulated color images. Digital information representing two sets of gray-level producing patterns, set A for color A and set B for color B, is electronically stored in computer memory. The patterns in set B are complementary to those of set A. An apparent or simulated color image is produced juxtapositioning a pattern from set A with a complementary pattern from set B, the combined image being subsequently rendered visible using two different colorants. A gray level pattern can be produced for each elemental area of an original image.
Tri-level xerography provides the ability to develop two different toners (typically different colors) in a document in a single pass of the charge retentive surface and copy substrate. Tri-level xerography is currently being used in the 4850.TM. machine to produce documents with black plus one highlight color at the full productivity of the base engine. In other words, the 4850.TM. machine produces prints at the rate of 50 copies per minute (cpm) whether it operates in the black only mode or in the highlight color mode. Unfortunately, Tri-level imaging is not applicable to process color printing when the single engine is expected to deliver two of the three primary colors in cyan, magenta and yellow. This is because process color images can demand up to 100% coverage, in an image, of both primary colors. For example, a saturated green would require complete coverage of both cyan and yellow. In tri-level imaging, each pixel must be one color or the other and cannot, therefore, contain both colors. That is, the total of both colors is 100% and green, for example, would be 50% cyan coverage and 50% yellow. Therefore, the resulting image would not be a saturated but rather a pale green. To circumvent this obstacle but achieve full productivity, numerous proposals for process color printing are based on tandem architectures, in which each process color separation is produced in a separate marking engine and the separations are recombined into a full color image through transfer to paper or another suitable intermediate. In principle, tandem architectures can include any number of engines (and, therefore colors) but typical configurations include three process primary color engines plus a black engine or a total of four engines.
It is an object of this invention to provide a color printing system using tri-level xerographic units to form process color images.
It is another object of this invention to provide a color printing system which can approximate a full color process.
It is still another object of this invention to provide a single pass color printing system which will not decrease productivity and which reduces the number and cost of optical and xerographic components.
Another object of the invention is to provide a full process color printer using spot on spot development whereby micro image registration requirements are not critical.
Yet another object of the present invention is to provide a full process color printer where image exposure is effected without having to form images by exposure through existing toner images.
Still yet another object of the present invention is to provide a full process color printer without development field degradation.